1. Field of the Invention
This invention relates to a method of determining voltage stability margin within an electrical power system, and an apparatus for monitoring voltage stability limits within an electrical power system and for dynamic control of load shedding and restoration.
2. Description of Related Art
During the large scale power system disturbances, the last line of defense which prevents the voltage collapse is the load shedding at the stations, where the voltage stability margin becomes too low.
These disturbances can lead to the electrical power system becoming unstable which can cause a voltage collapse or blackout.
In order to allow remedial action to be taken it is first necessary to establish when the electrical power system has become unstable.
Conventional monitoring systems compare electrical power system frequency, rate of frequency change, voltage level, rate of voltage decay, power flow levels, with a fixed reference value, and indicate that the power system has become unstable when the voltage, frequency has fallen below the reference value.
Today, the devices monitor the frequency and/or voltage with fixed settings at which the load shedding can be initiated in steps when the frequency or voltage has declined to an agreed-to level.
The disadvantage of such an approach results from the fact that the relation between the voltage level and the stability limit depends very much on the source EMF and the load power factor. Thus shedding the load in steps at fixed settings of voltage does not assure that the system remains stable after operation is completed.
Therefore there is a need to more accurately assess the level of voltage stability within an electrical power system to enable dynamic load shedding and restoration that is based on dynamic estimation of stability margin.